Mapping of the ocean bottom is carried on by means of ships utilizing depth sounding equipment. Such equipment is limited in that it only views the ocean bottom along the track of the ship in the vicinity or area directly beneath the ship. It is advantageous to utilize side looking sonar equipment to provide further coverage along both sides of the surveying ship, thereby to minimize the number of passes that a ship must make over a terrain to complete the mapping. A problem arises in the utilization of such side looking sonar equipment in that the sonar beams are directed at an angle to the vertical plane and thus intercept the ocean bottom at an angle thereby ensonifying a much larger and extended area of the ocean bottom by obliquely oriented beams than the area ensonified by a beam directed downwardly from the ship. These elongated ensonified regions provide an elongated echo of varying intensity which masks the true slant range between the ship and a point of reflection from the ocean bottom. Thus, measurements of ocean depth based on slant range measurements obtained by obliquely oriented sonar beams tend to be less accurate than those obtained from a depth sounder having its beam directed in the vertical plane at a point directly beneath the ship.